The present invention describes a method and an apparatus to replace a target zone in successive images of a video sequence, generally originating from a camera or a VCR, where the term "target zone" designates a rigid and identifiable zone.
Such methods and apparatus have already being described (U.S. Pat. No. 5,353,392 or WO-A-91/15921), applicable to a video stream taken in a studio environment as well as to images taken outdoors, notably on a stadium or on a motor racing circuit. The target zone (advertising billboard for instance) is automatically detected in the image using pattern recognition techniques, which use characteristic points or marks located within the target billboard, or next to it, or the stored shape of the zone. The characteristic points may be graphic elements in the target, lines of intensity or color discontinuities (boundaries for instance). The target may be required to have a specific color, blue or green, uniform or not in hue, and this color is then absent from the moving elements of the scene, so that occlusion by these moving parts can be handled by techniques known as "chroma-key" or "blue screen".
The target zone, once identified, is replaced in the video image by a representation of a predetermined model or "pattern" which has been pre-recorded, or by a video sequence. This representation is obtained by warping the model or the sequence so that it adapts to the target zone in such a way that it is seen in the video image at the location where the substitution must be performed.
For easier localization of the target billboard, U.S. Pat. No. 5,436,672 and EP-A-0,684,732 teach using predictive techniques and distinctive points distributed all over the images of the video stream to be modified.